1. Field of the Invention
The present invention relates to a selectively permeable hollow fiber membrane. More specifically, it provides a selectively permeable hollow fiber membrane which has a specific ultrafiltration rate and a specific overall mass transfer coefficient for dextran and sieving coefficient for albumin in dialyzers, has selective permeability to exhibit excellent separating performance, and which is suitable for such blood treatment as blood dialysis and blood filtration dialysis, being particularly suitable for removal of harmful substances in the medium high molecular weight range.
2. Description of the Related Art
Selectively permeable hollow fiber membranes have been used in common practice for reverse osmosis and blood dialysis. In particular, hollow fiber blood dialyzers are currently in wide use for purification of blood of kidney failure patients. Such dialyzers have a plurality of dialysis membranes, for example hollow fiber membranes in a cage-like housing, and the patient's blood is allowed to flow through the inner hollow while the dialyzing fluid is allowed to flow on the outside, i.e. between the hollow fibers, for dialysis through the hollow fiber membranes to remove waste products from the blood and rectify electrolyte concentrations, with ultrafiltration carried out by differential pressure between the inside and outside of the hollow fibers, to remove excess water from the blood. Hollow fibers are also sometimes used for treatment of autoimmune patients, by separating only the plasma from the blood, or by removing specific components from the plasma. Recently, therapeutic effects have been confirmed for the use of hollow fibers in protein-permeable blood dialysis and protein-permeable blood filtration dialysis.
Such hollow fibers for blood processing must be selectively permeable to specific substances, depending on the purpose. Their performance is determined based on the material of the hollow fibers as well as the porosity (size and number of pores, etc.) and membrane thickness.
Incidentally, recent years have seen an emphasis on the need for removal of harmful substances in the relatively medium high molecular weight range which are related to long-term complications in dialysis patients, such as .beta..sub.2 -MG (microglobulin) (molecular weight: 11,800) believed to be a causative substance of dialysis amyloidosis, parathyroid hormone (molecular weight: approximately 9,500) believed to be connected with itching sensation and hyperlipidemia, erythroblast inhibiting factor believed to be connected with anemia, and substances of molecular weight 20 to 40 thousand which are believed to be connected with arthralgia and ostealgia. On the other hand, loss of albumin (molecular weight: 66,000) which is essential to the human body must be avoided at all costs. In other words, there is a demand for selectively permeable membranes with a satisfactory sharp cut-off of molecular weight fractions, having excellent permeability to substances of molecular weight 40 to 50 thousand and lower while blocking substances of molecular weight 60,000 and greater.
However, although conventional polysulfone and other synthetic polymers, for example as in Japanese Examined Patent Publication No. 2-18695 and No. 5-54373, have been relatively satisfactory in terms of the demands mentioned above, in the case of cellulose derivatives, particularly triacetates such as in Japanese Examined Patent Publication No. 58-24165, in order to use substances such as liquid paraffin, higher alcohols and isopropyl myristate, which have no coagulability for triacetate spinning solutions, as the core materials for wet spinning of the hollow fibers, it is necessary to increase the triacetate concentration in the spinning solution for higher spinnability during spinning. Also, since hardening occurs with the coagulating solution on the outer surface of the hollow fibers after spinning the spinning solution from the nozzle, a dense structure layer forms on the outer surface of the hollow fibers.
For these reasons, conventional cellulose derivative hollow fibers have had a lower density difference between the structural dense layer and the porous layer in comparison to synthetic polymer membranes, almost to the point of becoming an entire uniform layer, and thus the substance permeability has been less than satisfactory. Attempts have therefore been made to improve the permeability by reducing the membrane thickness, but satisfactory results have not been achieved. In addition, since the difference between the dense layer and porous layer of the membrane structure is poorly defined, there has been much room for improvement in terms of the fractionating properties for the permeating substances.